(1) Field of the Invention
The present invention relates generally to the measurement of drag forces on a surface, and more particularly to a strut-mounted drag balance for measuring drag force on a flat plate moving through a medium.
(2) Description of the Prior Art
The performance of a vehicle moving through a medium is limited by the drag force of the medium on the vehicle. To improve efficiency and reduce the power required to move the vehicle through the medium, numerous drag reduction methods have been tried. It is customary to test drag reduction methods by model testing in towing basins or wind and water tunnels where a scaled model of the vehicle is towed through the medium or the medium is flowed past the model. To measure drag forces on the model, a towing post, or strut is attached to the model through a load cell, or drag balance. Typical prior art drag balances, as exemplified by U.S. Pat. No. 5,343,742 to Cusanelli et al., consist essentially of a hollow metal cube, generally four inches on a side, with portions of the sides removed, leaving planar top and bottom surfaces connected by four legs. The strut is attached to the top surface and the model is attached to the lower surface. Forces exerted on the model cause bending in the legs which is measured using strain gages attached to the legs. When the model is submerged in the medium such that the towing post extends into the medium, the strut is streamlined to minimize extraneous drag forces exerted on the strut, and the drag balance is placed within the scaled model. Testing of drag reduction methods which do not rely on vehicle shape to reduce drag, e.g., polymer injection and magneto hydrodynamic turbulence control devices, is complicated by the difficulty of predicting drag on a model shaped as a scaled vehicle. It is preferable to test such drag reduction methods using a flat plate model since drag calculations are much simpler for a flat plate model than for a hydrodynamically shaped model, thus actual test results can be easily compared to theoretical predictions. Since the flat plate has no interior volume, the drag balance cannot be placed within the plate. The cubic configuration of current drag balances would cause considerable turbulence if placed between the strut and the flat plate. If the drag balance were placed above the strut, the drag forces exerted on the strut itself would corrupt the drag reduction measurements. Additionally, The size of current drag balances also prevents their placement within an efficiently streamlined strut.